This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-275255, filed Sep. 11, 2001.
1. Field of the Invention
The invention relates to a surface treatment method of semiconductor substrate used in an electronic device having an SiGe/Si hetero junction.
2. Description of the Related Art
A semiconductor substrate utilizing germanium in formation of pn junction is preferably used in infrared detectors of high sensitivity and diodes. A SiGe substrate (or film) or Ge containing substrate (or film) incorporated in these electronic devices must be brought into contact with an Si film (or substrate). In this case, before the SiGe substrate (or film) or Ge containing substrate (or film) is brought into contact with the Si film (or substrate), it is important to clean the surface of the SiGe substrate (or film) or Ge containing substrate (or film) to remove impurities from the surface (impurity metal, oxide, carbide, etc.).
If surface cleaning of the SiGe substrate (or film) or Ge containing substrate (or film) is insufficient, impurity elements such as oxygen, carbon, and metal are left over on the surface, and when the Si film is deposited on the surface, stacking faults or point defects may be generated from these impurities. When the defect density on the pn junction boundary increases, it forms recombination centers, which cause the leak current in the pn junction when an inverse bias is applied to the pn junction. If a semiconductor substrate of such junction structure having a high defect density is incorporated in an electronic device (transistor or diode), probability of causing malfunction is high.
A hetero junction bipolar transistor (HBT) and a field effect transistor are known as electronic devices operating at high speed, and are widely used in communications and high speed signal processing. To further enhance the operation speed of such devices, a silicon-germanium film (SiGe film) is used.
In the HBT as shown in FIG. 6, if impurities are left over on the hetero junction boundary or surface of SiGe film 101, a leak current may be generated at the pn junction potential. Such leak current is one of the causes of failure of electronic device.
Impurities (foreign matters) left over on the SiGe surface or Ge surface are classified into three types as follows.
1) Oxides such as SiO2 and GeO2 
2) Carbon impurities such as hydrocarbon
3) Metal impurities such as Na, K, and Fe
Oxides are films naturally formed on the substrate surface when the surface of SiGe or Ge contacts with the atmosphere. Such oxide films are called natural oxide films, and the film thickness is about several nm to 20 nm.
Electronic devices using SiGe substrates or Ge substrates are manufactured in a device production plant using Si substrates. As chemical cleaning technology employed in a device production plant for Si, generally, the RCA method is known (W. Kern and D. A. Puotinen, RCA Vol. 31, 1970, 187).
The typical RCA method is executed in the following procedure (a to 1).
a. Cleaning in ultrapure water for several minutes.
b. Immersing in mixed solution of NH4OH, H2O2, and H2O (1:2:7) at 75xc2x0 C. for more than several minutes.
c. Cleaning in ultrapure water for several minutes.
d. Immersing in 1% hydrofluoric acid at room temperature for several minutes.
e. Cleaning in ultrapure water for several minutes.
f. Immersing in mixed solution of HCl, H2O2, and H2O (1:2:7) at room temperature for more than several minutes.
g. Cleaning in ultrapure water for several minutes.
h. Immersing in 1% hydrofluoric acid at room temperature for several minutes.
i. Cleaning in ultrapure water for several minutes.
j. Immersing in mixed solution of H2SO4, H2O2, and H2O (1:2:7) at room temperature for more than several minutes.
k. Cleaning in ultrapure water for several is minutes.
1. Drying by spin rotation.
Although the RCA method has been proved to be excellent for cleaning of Si substrate, but it is inappropriate for cleaning of SiGe substrate (or film) or Ge containing substrate (or film). That is because when the surface of SiGe substrate (or film) or Ge containing substrate (or film) is cleaned by RCA method, the surface is seriously roughened. If Si layer or metal layer is deposited on the SiGe substrate (or film) or Ge containing substrate (or film) thus having lost flatness, the defect incidence rate of final products of electronic devices is heightened, and the yield is lowered.
It is hence an object of the invention to present a surface treatment method of semiconductor substrate capable of removing impurities from the surface of a substrate having a surface layer composed of Ge or SiGe, achieving the flat surface and reducing the defect density on the hetero junction boundary.
The surface cleaning method of semiconductor substrate according to an aspect of the invention comprises (a) immersing a substrate having Ge or SiGe mixing Ge and Si at least in the surface layer in a solution of hydrofluoric acid, and removing impurities from the surface of the substrate without overetching the surface of the substrate, (b) pouring pure water on the substrate to wash away the solution of hydrofluoric acid applied in the step (a) from the surface of the substrate, and (c) immersing the substrate in a solution of hydrogen peroxide, and removing foreign matters from the surface of the substrate without overetching the surface of the substrate.
Further, a step (d) may be also a step of pouring pure water on the substrate to wash away the solution of hydrogen peroxide applied in the step (c) from the surface of the substrate, and removing the water applied on the surface of the substrate and then drying the substrate. On the dry surface of the substrate, a next thin film layer is deposited.
Prior to the step (a), preferably, the substrate is washed by running water. As a result, particle deposited on the surface are washed away from the substrate.
The solution of hydrofluoric acid used at step (a) preferably contains hydrofluoric acid by 1 mass % or more to 5 mass % or less. If the concentration of hydrofluoric acid exceeds 5 mass %, the surface of the substrate is overetched. On the other hand, if the concentration of hydrofluoric acid is less than 1 mass %, it takes too much time in processing, and impurities cannot be removed sufficiently. At this step (a), oxides such as SiO2 and GeO2 are removed from the surface of the substrate.
In the step (c), an oxide film of 1.0 nm or less in thickness is formed on the surface of the substrate. Such thin oxide film effectively prevents carbide foreign matters in the atmosphere (especially hydrocarbon) from sticking to the surface of the substrate.
In the step (b) or (d), a washing vessel is filled with pure water, the substrate is immersed in pure water in the washing vessel, and pure water is further supplied into the washing vessel while pure water is discharged from the washing vessel to form a flow of pure water in the washing vessel, and the surface of the substrate contacts with a running flow of pure water. As a result, the solution of hydrofluoric acid is removed from the substrate, and the solution of hydrofluoric acid is prevented from being mixed with the solution of hydrogen peroxide at the next step.
Further, putting substrates in a carrier, and arranging a vessel containing a solution of hydrofluoric acid, a vessel containing running flow of pure water, and a vessel containing a solution of hydrogen peroxide sequentially in series, preferably, in the step (a), the carrier containing the substrates is conveyed into the vessel of solution of hydrofluoric acid, the carrier containing the substrates is immersed in the solution of hydrofluoric acid, and the carrier containing the substrates is lifted from the solution of hydrofluoric acid, in the step (b), the carrier containing the substrates is transferred from the vessel of solution of hydrofluoric acid into the vessel of running flow of pure water, the carrier containing the substrates is immersed in the running flow of pure water, and the carrier containing the substrates is lifted from the running flow of pure water, and in the step (c) the carrier containing the substrates is transferred from the vessel of running flow of pure water into the vessel of solution of hydrogen peroxide, the carrier containing the substrates is immersed in the solution of hydrogen peroxide, and the carrier containing the substrates is lifted from the solution of hydrogen peroxide. By using such carrier, it is easier to immerse the substrates in and lift from the washing vessels in the continuous line, and the through-put of the treatment is substantially enhanced. It is preferred to contain a plurality (for example, 5 to 20) of substrates in the carrier, but it is possible to put only one substrate.
In the step (a), it is preferred to immerse the substrate in the solution of hydrofluoric acid at room temperature for at least one minute. At room temperature, however, when the substrate is immersed in the solution of hydrofluoric acid of, for example, 6% for more than 10 minutes, it causes overetching, and therefore, the substrate should not be immersed for a long time in a high-concentration solution of hydrofluoric acid.
In the step (b), preferably, the substrate is immersed in the running flow of pure water at room temperature for at least one minute.
In the washing steps (b) and (d) of substrate, it is recommended to use ultrapure water. Herein, ultrapure water refers to water satisfying the water quality standard of resistivity (specific resistance) of 13 Mxcexa9xc2x7cm or more, particle count of 30 or less per milliliter, and living bacteria count of 1 or less per milliliter.
Further, in order that the defect density on the hetero junction boundary may be 3xc3x971013/cm2 or less when a hetero junction is formed by depositing a Ge film or SiGe film mixing Ge and Si of second conductive type on the substrate, oxides, metal impurities, and carbon impurities are removed from the surface of the substrate in the steps (a) to (d). If the defect density increases on the hetero junction boundary, a leak current is generated on the junction boundary, and the basic function as electronic device deteriorates. For example, in the case of a diode, when the leak current increases, the breakdown voltage decreases, and a short circuit is formed in a worst case, and the diode fails to perform its basic rectifying action.
In the step (a), then substrate is cleaned in the solution of hydrofluoric acid, and oxides such as SiO2 and GeO2 are removed from the surface layer. Although SiGe or Ge on the surface is oxidized in the atmosphere in a short time, oxides formed by such oxidation are effectively removed in the step (a).
In the step (c), the substrate is cleaned in the solution of hydrogen peroxide, and the surface layer is slightly etched and oxidized. By cleaning in the step (c), carbide and metal impurities are removed from the surface layer of the substrate.
After the step (c), it is important to control the thickness of the oxide layer of the substrate surface layer at 1.0 nm or less, preferably 0.5 nm or less. Such a thin oxide layer can be easily removed by the subsequent heat treatment or reduction process. Such oxide layer effectively prevents hydrocarbon in the atmosphere from sticking again to the surface of the substrate when conveying the substrate taken out from the cleaning vessel.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.